The present invention relates to a control system for four-wheel drive vehicles, and more particularly to a system for controlling the torque capacity of a transfer clutch for four-wheel drive vehicles.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. Meanwhile, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" will occur at a sharp corner. This is caused by the front wheels running through an arc of greater radius than that of the rear wheels and therefore their tendency to rotate faster than the rear wheels, so that a large torsional torque occurs in the system. U.S. Pat. No. 4,576,061 discloses a system which is intended to eliminate the braking phenomenon. The system is provided with a steering angle sensor and a control system. The control system operates to convert a four-wheel drive power transmission operation to a two-wheel drive power transmission operation, when steering angle is larger than a set value at a sharp corner. However, although such a system prevents the tight corner braking at sharp corners by the two-wheel drive vehicle system, characteristics of the four-wheel drive vehicle such as driveability on slippery roads, uphill driving and other are not good. For example, when the vehicle negotiates a curve while the wheels of the vehicle slip on a snowy road, the transmission system is converted to a two-wheel drive in spite of slipping of wheels. As a result, the wheels slip more on the road. In addition, the steering angle sensor and the control system are complicated in construction.